Conventionally, in order to implement high-level automated operation in a plant or factory or the like, a distributed control system (DCS) has been constituted. Most of field devices (measuring instrument such as a pressure transmitter and actuator) that constitute the distributed control system perform communication via a wired communication bus cable. But, in recent years, wireless field devices are also realized that perform wireless communication.
Because wireless field devices may be installed in hazardous locations, such as a location in which a flammable gas is used, they must satisfy a safety standard for intrinsic safety and explosion-proofness. Specifically, they are designed so that internal electrical circuits (electronic circuits) do not act as a source of ignition for flammable gases, not only under normal conditions, but also in the event of foreseeable accidents. In addition, in order to satisfy a pressure resistance and explosion-proofness standard, it is necessary to adopt a pressure-resistant, explosion-proof construction that, even if an explosion occurs within a device, prevents flames due to the explosion from igniting a flammable gas outside of the device.
Following Patent Document 1 discloses a low-cost antenna apparatus that can be mounted in a fully enclosed container having a pressure-resistant, explosion-proof construction that is used in a hazardous location. With this antenna apparatus, in order to satisfy an intrinsic safety and explosion-proofness standard, two blocking capacitors are connected in series via a printed pattern and mounted on one surface of a printed circuit board housed in a housing, and two blocking capacitors are connected in series via a printed pattern and mounted on the other surface of the printed circuit board.